The present invention relates to a flexible riser installation for conveying hydrocarbons or other fluids under pressure and to a method of creating such an installation.
Flexible pipes for conveying hydrocarbons, as opposed to rigid pipes, are already well known and generally comprise, from the inside of the pipe outward, a metal carcass, to react the radial crushing forces, covered by an internal sealing sheath made of polymer, a pressure vault to withstand the internal pressure of the hydrocarbon, tensile armor layers to react axial tensile forces and a polymer external sheath to protect the entire pipe and in particular to prevent seawater from penetrating its thickness. The metal carcass and the pressure vault are made up of longitudinal elements wound with a short pitch, and give the pipe its ability to withstand radial force while the tensile armor layers consist of generally metal wires wound at long pitches in order to resist axial forces. It should be noted that in the present application, the idea of winding at a short pitch denotes any helical winding at a helix angle close to 90°, typically comprised between 75° and 90°. The idea of winding at a long pitch for its part covers helix angles of below 55°, typically comprised between 25° and 55° for the tensile armor layers.
These pipes are intended to convey hydrocarbons, particularly on the seabed, and to do so at deep depths. More specifically, they are said to be of the unbonded type and are thus described in the standards published by the American Petroleum Institute (API), API 17J and API RP 17B.
When a pipe, regardless of its structure, is subjected to an external pressure that is higher than the internal pressure, compressive forces directed parallel to the axis of the pipe and which tend to shorten the length of the pipe occur in the pipe wall. This phenomenon bears the name of reverse end cap effect. The intensity of the axial compressive forces is substantially proportional to the difference between the external pressure and the internal pressure. This intensity may reach very high levels in the case of a flexible pipe submerged at a great depth, because the internal pressure can, under certain conditions, be very much lower than the hydrostatic pressure.
In the case of a flexible pipe of conventional structure, for example one in accordance with the API standards, the reverse end cap effect has a tendency of introducing a longitudinal compressive force into the wires that make up the tensile armor layers, and to shorten the length of the flexible pipe. In addition, the flexible pipe is also subjected to dynamic bending stresses, particularly when it is being installed or when it is in service in the case of a riser, that is to say a pipe that makes the connection between a service installation at sea level or thereabouts, and an installation at the bottom of the sea. All of these stresses may cause the wires of the tensile armor layer to buckle and may irreversibly disorganize the tensile armor layers, thus destroying the flexible pipe.
Structural improvements to flexible pipes in order to increase the axial compressive strength of the armor layers have therefore been sought.
Thus, document WO 03/083343 describes such a solution which consists in winding around the tensile armor layers reinforced tapes, for example made with aramid fibers. This then limits and controls the expansion of the tensile armor layers. However, while this solution does solve the problems associated with the radial buckling of the wires that make up the tensile armor layers, it is capable only of limiting the risk of lateral buckling of said wires, which still remains.
Document WO 2006/042939 describes a solution which consists in using wires that have a high width-to-thickness ratio and in reducing the total number of wires that make up each tensile armor layer. However, while this solution reduces the risk of lateral buckling of the tensile armor layers, it does not completely eliminate it.
Application FR 06 07421 in the name of the Assignee hereof discloses a solution that involves adding to the inside of the structure of the flexible pipe a tubular axial-blocking layer. This layer is designed to react the axial compressive forces and to limit the shortening of the pipe, making it possible to avoid damaging the tensile armor layers.
These solutions are effective but have a certain number of constraints, particularly financial ones, which have led to a desire for alternative solutions, at least for specific cases, particularly for the specific case of risers.
There are various different configurations of flexible riser. The most widespread configurations are depicted in FIG. 4 of the standard “API RP 17B; Recommended Practice for Flexible Pipes; Third Edition; March 2002”. These are known to those skilled in the art by the names of “Free Hanging”, “Steep S”, “Lazy S”, “Steep Wave” and “Lazy Wave”. Another configuration, known by the name of “Pliant Wave®” is described in U.S. Pat. No. 4,906,137.
In the “Steep S”, “Lazy S”, “Steep Wave”, “Lazy Wave” and “Pliant Wave®” configurations, the flexible riser is supported, at a depth somewhere between the bottom and the surface, by one or more positive-buoyancy members, of the underwater buoy or arch type. This gives the flexible riser an S-shaped or wave-shaped geometry, allowing it to tolerate the vertical movements of the surface installation without introducing excessive curvature into said pipe, particularly in the region situated near to the seabed, as such excessive curvature is liable incidentally to damage said pipe. These configurations are generally reserved for dynamic applications at a depth of less than 500 m.
In the “Free Hanging” configuration the flexible riser is arranged as a catenary between the seabed and the surface installation. This configuration has the advantage of simplicity but the disadvantage of being ill-suited to dynamic applications at small depths because of the excessive variations in curvature that may be generated near the seabed. However, this configuration is commonly used for very deep applications, that is to say applications at depths in excess of 1000 m, or even 1500 m. This is because under such conditions, the relative amplitude of the movements of the floating support, particularly the vertical movements associated with the swell, remains very much smaller than the length of the catenary, thus limiting the amplitude of the variations in curvature near the seabed and making it possible to keep control over the risk of pipe fatigue and of lateral buckling of the tensile armor layers. However, in order to guarantee that the flexible pipe is able to withstand the reverse end cap effect, which at great depths may reach very high levels, the structure of the pipe has to be engineered according to the aforementioned known techniques, thus leading to solutions that are complex and expensive.
Also known are hybrid risers that use both rigid pipes and flexible pipes. Thus, documents FR 2 507 672, FR 2 809 136, FR 2 876 142, GB 2 346 188, WO 00/49267, WO 02/053869, WO 02/063128, WO 02/066786 and WO 02/103153 disclose a riser of the hybrid tower type. One or more rigid pipes rise up along a substantially vertical tower from the seabed up to a depth close to the surface, above which depth one or more flexible pipes provide the connection between the top of the tower and the floating support. The tower is equipped with buoyancy means to ensure that it remains in a vertical position. These hybrid towers are chiefly used for applications at very great depths. They have the disadvantage of being difficult to install. In particular, installing the rigid portion at sea generally requires very powerful lifting gear.
However, hitherto, no riser installation made as a flexible pipe standing vertically and able effectively to withstand the reverse end cap effect in uses in deep seas (that is to say typically at depths in excess of 1000 m, or even 1500 or 2000 m) without recourse to expensive structural modifications to the pipe was known. At such great depths, the end cap effect has a very large amplitude because of the magnitude of the hydrostatic pressure. When, in an installation for conveying hydrocarbons, particularly in gaseous form, production is halted, for example by closing a valve, the internal pressure inside the pipe may drop and the difference between the high external hydrostatic pressure and the low or zero internal pressure may become considerable. It is conditions such as these that give rise to the reverse end cap effect. If it is desired that a flexible pipe be used in a conventional riser installation, then it is obligatory to adapt the structure of the pipe so that it can withstand the reverse end cap effect at the foot of the riser, which means engineering the pipe reinforcing layers accordingly, the foot of the riser being the determining part, which leads to the remainder of the pipe being overengineered and therefore leads to additional cost.